Time-lapse morphological analysis of human cells under THz irradiation shows inhibition of cell division at cytokinesis. The contractile ring, which must disappear at the end of cell division remained for 1 hour under THz irradiation. This phenomenon was not reproduced by heating up the sample, indicating that this is non-thermal effect of the THz irradiation. We also observed induction of filamentous actin, which is the major component of contractile ring, during THz irradiation (460 GHz and 280 GHz). These results are in good agreement with previous in vitro experiments in which polymerization of actin protein was enhanced by THz irradiation.